1. Field of the Invention
The present invention relates to an imageable element comprising a graft copolymer having hydrophilic and hydrophobic segments. More particularly, the present invention relates to a graft copolymer, which upon imagewise exposure to thermal energy, produces exposed regions that are less soluble in a developer than the unexposed regions.
2. Description of the Prior Art
Thermally imageable compositions employing hydrophobic heat sensitive polymer particles dispersed in hydrophilic binders are generally known in the art.
JP 60/68992 and EP 514 145 disclose the use of core-shell particles in a thermally imageable composition that can undergo at least partial thermal coalescence.
EP 599 510 and equivalent U.S. Pat. No. 5,948,599 disclose a method of forming an image using a radiation sensitive plate, which comprises a layer which includes (1) a reactive group that undergoes insolubilization at elevated temperature and/or exposure to radiation, and (2) a substance capable of strongly absorbing radiation to produce heat. The layer contains a disperse phase, which has a heat softenable component and a binder or continuous phase that has a component that is soluble or swellable in an aqueous medium. Radiation causes at least partial coalescence in the exposed regions. Following development, the plate is heated or irradiated to effect insolubilization of the image areas by reaction of the reactive groups.
EP 816 070 discloses hydrophobic thermoplastic particles, photothermal converter and a covering layer.
U.S. Pat. No. 6,071,369 and EP 839 647 disclose a method for imagewise exposing an image forming layer that has thermoplastic hydrophobic polymer particles in a hydrophilic binder. Laser exposure is limited to a pixel dwell time of 0.1 to 50 microseconds.
U.S. Pat. No. 6,022,667 and EP 881 096 disclose an imageable layer comprising a hydrophobic thermoplastic polymer latex and a photothermal converter. These references require the use of an alkali soluble intermediate layer between the substrate and the imageable layer.
None of the prior art discloses heat sensitive graft copolymers of the present invention.
In broad concept, the present invention includes an imageable element comprising a substrate and a thermally imageable composition coated on a surface of the substrate, wherein the thermally imageable composition comprises a graft polymer having hydrophilic and hydrophobic segments. The imageable element according to the present invention is thermally imageable.
The present invention includes an imageable element comprising: a substrate; and a thermally imageable composition coated on a surface of the substrate; wherein the thermally imageable composition comprises a graft copolymer having a hydrophobic polymer backbone and a plurality of pendant groups represented by the formula:
xe2x80x94Qxe2x80x94Wxe2x80x94Y
wherein Q is a difunctional connecting group; W is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment; Y is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment; with the proviso that when W is a hydrophilic segment, Y is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment, with the further proviso that when W is hydrophobic, Y is a hydrophilic segment.
The present invention also includes a method of producing an imaged element. The method comprises the steps of:
providing an imageable element comprising: a substrate; and a thermally imageable composition coated on a surface of the substrate; the thermally imageable composition comprising a graft copolymer having a hydrophobic polymer backbone and a plurality of pendant groups represented by the formula:
xe2x80x94Qxe2x80x94Wxe2x80x94Y
wherein Q is a difunctional connecting group; W is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment; Y is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment; with the proviso that when W is a hydrophilic segment, Y is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment, with the further proviso that when W is hydrophobic, Y is a hydrophilic segment;
imagewise exposing the imageable element to thermal radiation to produce exposed and unexposed regions; and
contacting the imagewise exposed imageable element and a developer to remove the unexposed regions and thereby produce the imaged element.
The present invention also includes a thermally imageable composition comprising a graft copolymer having a hydrophobic polymer backbone and a plurality of pendant groups represented by the formula:
xe2x80x94Qxe2x80x94Wxe2x80x94Y
wherein Q is a difunctional connecting group; W is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment; Y is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment; with the proviso that when W is a hydrophilic segment, Y is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment, with the further proviso that when W is hydrophobic, Y is a hydrophilic segment.
The graft copolymers of present invention can be imaged without a binder. Furthermore, the thermally imageable compositions can be imaged and developed without the need of an intermediate pre-heat step. In addition, the present invention does not require the use of an intermediate barrier or a covering layer.
The present invention provides a thermally imageable composition that is useful in thermal imaging of, for example, lithographic plates and printed circuit boards.
Lithographic printing is based on the immiscibility of oil and water. Ink receptive areas are generated on the surface of a hydrophilic surface. When the surface is moistened with water and then ink is applied, the hydrophilic background areas retain the water and repel the ink. The ink receptive areas accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is thereafter reproduced.
Lithographic printing plate precursors, i.e., imageable elements, typically include a radiation-sensitive coating applied over the hydrophilic surface of a support material. If after imagewise exposure to radiation, the exposed regions of the coating become the ink-receptive image regions, the plate is called a negative-working printing plate. Conversely, if the unexposed regions of the coating become the ink-receptive image regions, the plate is called a positive-working plate. In the present invention, the imagewise exposed regions are rendered less soluble or dispersible in a developer and become the ink-receptive image areas. The unexposed regions, being more readily soluble or dispersible in the developer, are removed in the development process, thereby revealing a hydrophilic surface, which readily accepts water and becomes the ink-repellant non-image area.
The present invention includes a thermally imageable composition comprising a graft copolymer, preferably a thermally sensitive graft copolymer, having hydrophobic and hydrophilic segments. The thermally imageable composition comprises a graft copolymer having, preferably, a hydrophobic polymer backbone and a plurality of pendant groups represented by the formula:
xe2x80x94Qxe2x80x94Wxe2x80x94Y
wherein Q is a difunctional connecting group;
W is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment;
Y is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment;
with the proviso that when W is a hydrophilic segment, Y is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment, with the further proviso that when W is hydrophobic, Y is a hydrophilic segment.
The term xe2x80x9cgraftxe2x80x9d polymer or copolymer in the context of the present invention refers to a polymer which has as a side chain a group having a molecular weight of at least 200. Such graft copolymers can be obtained, for example, by anionic, cationic, non-ionic, or free radical grafting methods, or they can be obtained by polymerizing or co-polymerizing monomers, which contain such groups.
The term xe2x80x9cpolymerxe2x80x9d in the context of the present invention refers to high and low molecular weight polymers, including oligomers, and includes homopolymers and copolymers. The term xe2x80x9ccopolymerxe2x80x9d refers to polymers that are derived from two or more different monomers.
The term xe2x80x9cbackbonexe2x80x9d in the context of the present invention refers to the chain of atoms in a polymer to which a plurality of pendant groups are attached. An example of such a backbone is an xe2x80x9call carbonxe2x80x9d backbone obtained from the polymerization of an olefinically unsaturated monomer.
The term xe2x80x9chydrocarbylxe2x80x9d in the context of the present invention refers to a linear, branched or cyclic alkyl, alkenyl, aryl, aralkyl or alkaryl of 1 to 120 carbon atoms, and substituted derivatives thereof. The substituent group can be halogen, hydroxy, acyloxy, hydrocarbyloxy, carboxyl, ester, ketone, cyano, amino, amido and nitro groups. Hydrocarbyl groups in which the carbon chain is interrupted by oxygen, nitrogen or sulfur are also included in the term xe2x80x9chydrocarbylxe2x80x9d.
The term xe2x80x9chydrocarbylenexe2x80x9d in the context of the present invention refers to a linear, branched or cyclic alkylene, vinylene, arylene, aralkylene or alkarylene of 1 to 120 carbon atoms, and substituted derivatives thereof. The substituent group can be halogen, hydroxy, acyloxy, hydrocarbyloxy, carboxyl, ester, ketone, cyano, amino, amido and nitro groups. Hydrocarbylene groups in which the carbon chain is interrupted by oxygen, nitrogen or sulfur are also included in the term xe2x80x9chydrocarbylenexe2x80x9d.
The present invention includes a graft copolymer comprising repeating units represented by the formula: 
wherein each of R1 and R2 is independently selected from the group consisting of: H, alkyl, aryl, aralkyl, alkaryl, COOR5, R6CO, halogen and cyano;
Q is selected from the group consisting of: 
wherein R3 is selected from the group consisting of: H and alkyl; R4 is selected from the group consisting of: H, alkyl, halogen, cyano, nitro, alkoxy, alkoxycarbonyl, acyl and a combination thereof;
W is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment;
Y is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment;
Z is selected from the group consisting of: H, alkyl, halogen, cyano, acyloxy, alkoxy, alkoxycarbonyl, hydroxyalkyloxycarbonyl, acyl, aminocarbonyl, aryl and substituted aryl;
j is at least 1;
k is at least 1; and
m is at least 2;
with the proviso that when W is a hydrophilic segment, Y is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment, with the further proviso that when W is hydrophobic, Y is a hydrophilic segment.
In one embodiment, the preferred graft copolymer of the present invention comprises main chain segments that are predominately hydrophobic and branch segments that are predominately hydrophilic.
In a second embodiment, the preferred graft copolymer comprises main chain segments that are predominately hydrophobic and branch segments comprising both hydrophobic and hydrophilic segments.
The hydrophilic segment in W in the graft copolymer of the present invention is preferably a segment represented by the formula: 
wherein each of R7, R8, R9 and R10 can independently be H or methyl; R3 can be H or alkyl; and the hydrophobic segment in W can be xe2x80x94R12xe2x80x94, xe2x80x94Oxe2x80x94R12xe2x80x94Oxe2x80x94, xe2x80x94R3Nxe2x80x94R12xe2x80x94NR3xe2x80x94, xe2x80x94OOCxe2x80x94R12xe2x80x94Oxe2x80x94 or xe2x80x94OOCxe2x80x94R12xe2x80x94Oxe2x80x94, wherein each R12 can independently be a linear, branched or cyclic alkylene of 6-120 carbon atoms, a haloalkylene of 6-120 carbon atoms, an arylene of 6-120 carbon atoms, an alkarylene of 6-120 carbon atoms or an aralkylene of 6-120 carbon atoms; and R3 can be H or alkyl; and wherein the hydrophilic segment in Y can be H, R15, OH, OR16, COOH, COOR16, O2CR16, a segment represented by the formula: 
wherein each of R7, R8, R9 and R10 can independently be H and methyl; R3 can be H or alkyl; wherein each R13, R14, R15 and R16 can independently be H oralkyl of 1-5 carbon atoms and wherein the hydrophobic segment can be a linear, branched or cyclic alkyl of 6-120 carbon atoms, a haloalkyl of 6-120 carbon atoms, an aryl of 6-120 carbon atoms, an alkaryl of 6-120 carbon atoms, an aralkyl of 6-120 carbon atoms, OR17, COOR17 or O2CR17, wherein R17 is an alkyl of 6-20 carbon atoms; and wherein n is from about 5 to about 250.
In a preferred embodiment, the graft copolymer comprises repeating units represented by the formula: 
wherein each of R1 and R2 can independently be H, alkyl, aryl, aralkyl, alkaryl, COOR5, R6CO, halogen or cyano.
Q can be one of: 
wherein R3 can be H or alkyl; R4 can independently be H, alkyl, halogen, cyano, nitro, alkoxy, alkoxycarbonyl, acyl or a combination thereof.
The segment W can be a hydrophilic segment or a hydrophobic segment, wherein the hydrophilic segment can be a segment represented by the formula: 
wherein each of R7, R8, R9 and R10 can independently be H or methyl; R3 can be H and alkyl; and wherein the hydrophobic segment can be xe2x80x94R12xe2x80x94, xe2x80x94Oxe2x80x94R12xe2x80x94Oxe2x80x94, xe2x80x94R3Nxe2x80x94R12xe2x80x94NR3xe2x80x94, xe2x80x94OOCxe2x80x94R12xe2x80x94Oxe2x80x94 or xe2x80x94OOCxe2x80x94R12xe2x80x94Oxe2x80x94, wherein each R12 can independently be a linear, branched or cyclic alkylene of 6-120 carbon atoms, a haloalkylene of 6-120 carbon atoms, an arylene of 6-120 carbon atoms, an alkarylene of 6-120 carbon atoms or an aralkylene of 6-120 carbon atoms; R3 can be H or alkyl.
Y can be a hydrophilic segment or a hydrophobic segment, wherein the hydrophilic segment can be H, R15, OH, OR16, COOH, COOR16, O2CR16, a segment represented by the formula: 
wherein each of R7, R8, R9 and R10 can independently be H or methyl; R3 can be H and alkyl; wherein each R13, R14, R15 and R16 can be H or alkyl of 1-5 carbon atoms and wherein the hydrophobic segment can be a linear, branched or cyclic alkyl of 6-120 carbon atoms, a haloalkyl of 6-120 carbon atoms, an aryl of 6-120 carbon atoms, an alkaryl of 6-120 carbon atoms, an aralkyl of 6-120 carbon atoms, OR17, COOR17 or O2CR17, wherein R17 can be an alkyl of 6-20 carbon atoms.
Z can be H, alkyl, halogen, cyano, acyloxy, alkoxy, alkoxycarbonyl, hydroxyalkyloxycarbonyl, acyl, aminocarbonyl, aryl and substituted aryl;
j is at least 1;
k is at least 1;
m is at least 2; and
n is from about 5 to about 250; with the proviso that when W is a hydrophilic segment, Y is a hydrophilic segment or a hydrophobic segment, with the further proviso that when W is hydrophobic, Y is a hydrophilic segment. The substituent in the above substituted aryl can be alkyl, halogen, cyano, alkoxy or alkoxycarbonyl. Preferably, the alkyl group is an alkyl of 1 to 22 carbon atoms.
In another preferred embodiment, the segment W-Y can be represented by the formula:
xe2x80x94(OCH2CH2)nxe2x80x94OCH3
wherein n is from about 10 to about 75. In this preferred embodiment, the graft copolymer has, for example, repeating units represented by the formula: 
wherein j and k are each at least 1; m is at least 5; and n is from about 10 to about 75. More preferably, n has an average value of about 45.
In another preferred embodiment, the graft copolymer comprises repeating units represented by the formula: 
wherein j and k are each at least 1; m is at least 5; and n is from about 10 to about 75, more preferably, n has an average value of about 45.
The graft copolymer having hydrophobic and/or hydrophilic segments is prepared by process comprising the steps of: contacting, in the presence of a catalyst:
(i) a compound represented by the formula:
Hxe2x80x94Wxe2x80x94Y
wherein W is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment and Y is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment, with the proviso that when W is a hydrophilic segment, Y is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment, with the further proviso that when W is hydrophobic, Y is a hydrophilic segment; and
(ii) a polymerizable monomer selected from the group consisting of compounds represented by the formula: 
wherein each R1 is independently selected from the group consisting of: H, alkyl, aryl, aralkyl, alkaryl, COOR5, R6CO, halogen and cyano; R4 is selected from the group consisting of: H, alkyl, halogen, cyano, nitro, alkoxy, alkoxycarbonyl, acyl and a combination thereof; and X is glycidyloxy or a leaving group selected from the group consisting of: halogen, alkoxy or aryloxy, to produce a polymerizable graft monomer; and
copolymerizing the polymerizable graft monomer and one or more comonomers at a temperature and for a period of time sufficient to produce the graft copolymer.
Preferably, the comonomer is one or more of the following: styrene, substituted styrene, alpha-methylstyrene, acrylate ester, methacrylate ester, acrylonitrile, acrylamide, methacrylamide, vinyl halide, vinyl ester, vinyl ether and an alpha-olefin.
The preferred polymerizable monomer can be any monomer that is capable of reacting with Hxe2x80x94Wxe2x80x94Y and include polymerizable monomers, such as, m-isopropenyl-xcex1, xcex1-dimethylbenzyl isocyanate, acryloyl chloride and methacryloyl chloride. The reaction is typically carried out in the presence of a catalyst, which is preferably a base, a tin compound or a mixture thereof. In a reaction that admits to an acid catalyst, an acid catalyst such as a Lewis or protic acid may be used.
Preferably, the compounds represented by the formula Hxe2x80x94Wxe2x80x94Y can be one or more of compounds represented by the formula: 
wherein each of R7, R8, R9 and R10 can independently be H or methyl; R3 can be H or alkyl; Y can be alkyl, acyloxy, alkoxy or carboxylate; and n is from about 5 to about 250.
The graft copolymer is typically obtained by a free-radical copolymerization of the graft monomer and the comonomer, preferably at a comonomer to graft monomer weight ratio of from about about 99:1 to about 45:55.
Alternatively, the graft copolymer can be prepared by first copolymerizing a polymerizable monomer according to the present invention with one or more comonomers at a temperature and for a period of time sufficient to produce a graftable copolymer and thereafter grafting the group xe2x80x94Wxe2x80x94Y onto the graftable copolymer. Such grafting can be achieved by contacting in the presence of a catalyst the above graftable copolymer and a compound represented by the formula:
Hxe2x80x94Wxe2x80x94Y
wherein W can be a hydrophilic segment or a hydrophobic segment and Y can be a hydrophilic segment and a hydrophobic segment, with the proviso that when W is a hydrophilic segment, Y is either a hydrophilic segment or a hydrophobic segment, with the further proviso that when W is hydrophobic, Y is a hydrophilic segment.
Preferably, the graft copolymer has Number Average Molecular Weight of the graft copolymer is from about 2,000 to about 2,000,000 and a glass transition temperature of at least 40xc2x0 C., more preferably, the graft copolymer has a glass transition temperature of from about 50xc2x0 C. to about 150xc2x0 C.
In a preferred embodiment, the present invention also includes a thermally imageable composition comprising a graft copolymer comprising repeating units represented by the formula: 
wherein each of R1 and R2 is independently selected from the group consisting of: H, alkyl, aryl, aralkyl, alkaryl, COOR5, R6CO, halogen and cyano;
Q is selected from the group consisting of: 
wherein R3 is selected from the group consisting of: H and alkyl; R4 is selected from the group consisting of: H, alkyl, halogen, cyano, nitro, alkoxy, alkoxycarbonyl, acyl and a combination thereof;
W is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment;
Y is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment;
Z is selected from the group consisting of: H, alkyl, halogen, cyano, acyloxy, alkoxy, alkoxycarbonyl, hydroxyalkyloxycarbonyl, acyl, aminocarbonyl, aryl and substituted aryl;
j is at least 1;
k is at least 1; and
m is at least 2;
with the proviso that when W is a hydrophilic segment, Y is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment, with the further proviso that when W is hydrophobic, Y is a hydrophilic segment.
The thermally imageable composition comprising the above described graft copolymer according to the present invention can be used in imaging applications with or without the use of a separate binder, i.e., a binder is not essential, so that the thermal imaging can be carried out without a binder. However, the thermal imaging process may be facilitated by the use of a binder. Accordingly, the thermally imageable composition of the present invention can further comprise a binder.
The present invention is useful in negative-working printing plates. Preferably, imaging is carried out using an infrared laser and a photothermal converter for converting IR radiation into heat. However, UV and visible laser imaging may also be used together with an appropriate photothermal converter. Accordingly, the thermally imageable composition of the present invention can further comprise a photothermal converter material, i.e., a material that is capable of converting electromagnetic radiation into heat.
The present invention further includes an imageable element and a method of producing an imaged element using the imageable element. In a preferred embodiment, the imageable element comprises a substrate; and a thermally imageable composition coated on a surface of the substrate; wherein the thermally imageable composition comprises a graft copolymer comprising repeating units represented by the formula: 
wherein each of R1 and R2 is independently selected from the group consisting of: H, alkyl, aryl, aralkyl, alkaryl, COOR5, R6CO, halogen and cyano;
Q is selected from the group consisting of: 
wherein R3 is selected from the group consisting of: H and alkyl; R4 is selected from the group consisting of: H, alkyl, halogen, cyano, nitro, alkoxy, alkoxycarbonyl, acyl and a combination thereof;
W is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment;
Y is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment;
Z is selected from the group consisting of: H, alkyl, halogen, cyano, acyloxy, alkoxy, alkoxycarbonyl, hydroxyalkyloxycarbonyl, acyl, aminocarbonyl, aryl and substituted aryl;
is at least 1;
k is at least 1; and
m is at least 2;
with the proviso that when W is a hydrophilic segment, Y is selected from the group consisting of: a hydrophilic segment and a hydrophobic segment, with the further proviso that when W is hydrophobic, Y is a hydrophilic segment.
The substrate of the imageable element is typically an aluminum sheet. However, other materials that are commonly known to those skilled in the art can also be used. Suitable substrates include any sheet material conventionally used to prepare lithographic printing plates, including metals such as aluminum sheets; paper; paper coated on one or both sides with an .alpha.-olefin polymer such as polyethylene; films such as cellulose acetate film, polyvinyl acetal film, polystyrene film polypropylene film, polyester film such as polyethylene terephthalate film, polyamide film, polyimide film, nitrocellulose film, polycarbonate film, polyvinylchloride film; composite films such as polyester, polypropylene or polystyrene film coated with polyethylene film; metalized paper or films; metal/paper laminates and the like.
The surface of plastic films may be treated using the surface treatment techniques known in the art to improve adhesion between the substrate and organic coatings.
A preferred substrate is an aluminum sheet. The surface of the aluminum sheet may be treated with metal finishing techniques known in the art including physical roughening, electrochemical roughening, chemical roughening, anodizing, and silicate sealing and the like. If the surface is roughened, the average roughness (Ra) is preferably in the range from 0.1 to 0.8 xcexcm, and more preferably in the range from about 0.1 to about 0.4 xcexcm. The preferred thickness of the aluminum sheet is in the range from about 0.005 inch to about 0.020 inch.
The preferred substrate is electrochemically-grained and anodized aluminum, such as commonly used for lithographic printing plates.
Anodic pore size for sulfuric acid anodization is typically less than 20 nm whereas anodic pore size for phosphoric acid anodization is typically greater than 30 nm. The use of large anodic pore substrates that are phosphoric acid anodized is preferred over sulfuric acid-anodized substrates. Other conventional anodization methods can also be used in the preparation of the anodized substrate of the present invention, including particularly those that produce an anodic pore size larger than anodic pore size produced by sulfuric acid anodization.
The graft copolymer can be applied onto the substrate as a solution or dispersion in the coating liquid of the image-forming layer by a suitable coating method. Illustrative of such a method is dissolving the graft copolymer in an organic water immiscible solvent, dispersing the resulting solution in an aqueous medium, applying the resulting dispersion onto a substrate and thereafter removing the solvent by evaporation. After proper drying, the coating weight of the layer is preferably in the range of about 0.2 to about 5.0 g/m2, and more preferably in the range from about 0.7 to about 2.5 g/m2.
The amount of the graft copolymer in the image-forming layer is preferably at least 30% by weight and more preferably at least 45% by weight and most preferably at least 60% by weight.
The photothermal converter material is preferably included in the image-forming layer but can also be provided in a layer adjacent to the image-forming layer. Suitable compounds capable of converting light into heat are preferably infrared absorbing components. Particularly useful photothermal converters are infrared absorbing dyes and pigments. Classes of materials that are useful as photothermal converters include, but are not limited to, squarylium, croconate, cyanine (including phthalocyanine), merocyanine, chalcogenopyryloarylidene, bis (chalcogenopyrylo) polymethine, oxyindolizine, quinoid, indolizine, pyrylium and metal thiolene dyes or pigments. Other useful classes include thiazine, azulenium and xanthene dyes. Still other useful classes are carbon blacks, metal carbides, borides, nitrides, carbonitrides and bronze-structured oxides. Particularly useful as photothermal converters are infrared absorbing dyes of the cyanine class.
The amount of infrared absorbing compound in the image forming layer is generally sufficient to provide an optical density of at least 0.5 and, preferably, an optical density of from about 1 to about 3 in the layer. This range would accommodate a wide variety of compounds having different extinction coefficients. Generally, the amount of infrared absorbing compound in the image forming layer is at least 1 weight percent and, preferably, it is from about 5 to about 30 weight percent.
The method of producing an imaged element comprises the steps of:
providing an imageable element comprising a substrate and a thermally imageable composition comprising a graft copolymer according to the present invention coated on a surface of the substrate;
imagewise exposing the imageable element to thermal radiation to produce exposed and unexposed regions; and
contacting the imagewise exposed imageable element and a developer to remove the unexposed regions and thereby produce the imaged element.
Imagewise exposure of the graft copolymer to thermal energy renders the exposed regions less soluble in a developer, which is preferably an aqueous developer. The imageable element according to the present invention is thermally imageable and becomes less soluble in a developer upon exposure to thermal radiation.
Without being bound by theory, it is believed that heating the graft copolymers of the present invention, which contain both hydrophobic and hydrophilic segments, causes realignment of the ink-receptive hydrophobic segments such that the hydrophobic segments are exposed to the coating surface.
Such realignment of the ink-receptive hydrophobic segments is possible because exposure to heat would cause a heat-softening of the graft copolymer, which facilitates mobility of the ink-receptive hydrophobic segments to the coating surface. Such a realignment of the hydrophobic segments would also be facilitated by the generally lower surface energies of the hydrophobic segments than those of the hydrophilic segments.
Realignment of the hydrophobic segments on the surface of the coating reduces the solubility of the thermally exposed regions in the aqueous developer and thereby provides an ink-receptive image area in the exposed areas.
The imageable element according to the present invention is thermally imageable. Upon such imaging the developer solubility of the imaged area is decreased to allow differentiation between imaged and non-imaged areas upon development. Typically, the step of exposing the imageable element to thermal radiation is carried out using an infrared laser. However, other methods such as visible or UV laser imaging may also be used, provided that a photoconverter, i.e., a photothermal converter, is present. Thus, for exposure with such visible or UV radiation sources, the imageable composition generally includes a photothermal converting material. Alternatively, the imageable element of the present invention can be imaged using a conventional apparatus containing a thermal printing head or any other means for imagewise conductively heating the imageable composition, such as, with a heated stylus or with a heated stamp.
The aqueous developer composition is dependent on the nature of the graft copolymer composition. Common components of aqueous developers include surfactants, chelating agents, such as salts of ethylenediamine tetraacetic acid, organic solvents, such as benzyl alcohol, and alkaline components, such as, inorganic metasilicates, organic metasilicates, hydroxides and bicarbonates. The pH of the aqueous developer is preferably within about 5 to about 14, depending on the nature of the graft copolymer composition.
Following development, a postbake may optionally be used to increase press life.
In addition to the thermally imageable layer, the thermally imageable element can have additional layers, such as, an underlying layer. Possible functions of an underlying layer include:
(1) to enhance developability of the imagewise unexposed areas; and
(2) to act as a thermal insulating layer for the imagewise exposed areas.
Such a thermal insulating polymeric layer prevents otherwise rapid heat dissipation, for example, through the heat conducting aluminum substrate. This allows more efficient thermal imaging throughout the thermally imageable layer, particularly in the lower sections. In accordance with these functions, the underlying layer should be soluble or at least dispersible in the developer and, preferably, have a relatively low thermal conductivity coefficient.
The thermally imageable element can further have an overlying layer. Possible functions of an overlying layer include:
(1) to prevent damage, such as scratching, of the surface layer during handling prior to imagewise exposure; and
(2) to prevent damage to the surface of the imagewise exposed areas, for example, by over-exposure which could result in partial ablation. The overlying layer should be soluble, dispersible or at least permeable to the developer.
The invention is further described in the following examples, which are intended to be illustrative and not limiting.